The non-pressure thermosiphon systems rely on the nature circulation of the water between solar tubes and the water tank. Thermosiphon systems work on principal of heat rising. The water enters in the bottom of the collector and rises to the tank as it warms. The advantage of this system is that solar heat is stored in a well-insulated tank, so hot water can be used at any time without the penalty of overnight losses. Moreover the insulation of the tank can protect heat during 3 days without heat loss.